1. Field of the Invention
The present invention relates to tripods of the type used for supporting video or photo equipment and, more particularly, relates to an assembly for applying controlled friction for smooth operation of the rack and pinion mechanism which raises and lowers the center column of the tripod.
2. State of the Prior Art
Conventional tripods of the type used for supporting a camera and other video or photographic equipment have telescopically extendable legs which at their upper end are pivotally attached to a hub body which remains stationary and is supported on the tripod legs. A center column of the tripod moves axially through a bore in the hub. A hand operated crank is mounted to the hub and turns a pinion engaged to a gear rack which extends along the center column, for raising and lowering the column through the hub. The upper end of the center column supports a panning head on which a camera can be mounted. The height of the camera above a ground surface can be continuously adjusted by raising or lowering the center column of the tripod.
The center column can be fixed at a particular position relative to the hub by means of a detent mechanism. One known expedient for preventing unwanted vertical movement of the center column employs nearly cylindrical sleeves made of synthetic resin and mounted in the cylindrical bore through which moves the center column, providing a fixed resistance to sliding motion of the center column through the sleeves caused by friction between the inner circumferential surface of the sleeves and the outer circumferential surface of the center column. A drawback has been that after prolonged use of the tripod, the synthetic resin sleeves tend to deform or creep, resulting in a reduced resistance and support of the center column.
One attempt to overcome this problem is described in No. 11875 of the 1975 Official Utility Model Gazette of the Japanese Patent Office, and in No. 8194 of the 1991 Official Patent Gazette of the same patent office, in which a tripod is provided with the capability of adjusting the resistance to sliding motion of the center column by use of sleeves with ends which become narrower towards the top, and a nut which screws coaxially to the cylindrical bore in the hub body, altering the friction between the interior surface of the sleeves and the outer surface of the center column by displacing the inclined surfaces of the sleeves in a radial direction by turning and advancing the nut. The aforementioned tripod has a gear rack which extends in the axial direction on a side surface of the center pole, and by rotating a pinion mounted on the hub, the center column can be moved smoothly up and down through the hub.
This prior art mechanism suffers from the disadvantage that it is necessary to properly adjust the screw tension of the nut which tightens the sleeves for adjusting the resistance to motion of the center pole, and consequently problems may be caused by improper adjustment of the mechanism. If the nut is loosened while the tripod is in use, the center column will not be stably supported, and improper engagement between the rack and pinion may occur, possibly resulting in damage to the rack and/or the pinion.
The present invention addresses the aforementioned shortcomings of the prior art, and aims to provide an easy to use tripod in which an appropriate resistance to the motion of the center column is reliably maintained.